To get Saibamen you have to have Dragon Arena and your character has to be level 10-20 and you need to fight some one at least twice your level and Nappa Should break-in. Defeat him to get Saibamen. To get Cell Jr. you also have to have Dragon Arena and your character has to be level 20-40 and Piccilo will eventually break-in. Defeat him to get Cell Jr.

to get Videl, you have to go to the lower islands as Gohan. you can just go to playstation-2.cheats.it (Italian version written in english, but some of the cheats still work, including Supreame Kai'scheat)

To get Videl in Gohan's DU: Defeat Goten first (not necessary but it would affect in gettingSupreme Kai if you get it prior fighting Goten...)Proceed to the ??? marker located in the plains between Goku'sHouse and Grandpa Gohan's houseGet her capsule at the "Plains" marker located at bottom right corner of the map.To get Supreme Kai in Gohan's DU: Complete Gohan's DU at least once. Get Videl's capsule (see instructions above)Make your way to West City and use Ki Search. Look for a "Plains" marker, which is located northeast off the city. You will fight Vegeta. Defeat him Continue the story and you will battle Majin Vegeta instead of Dabura. Defeat him. Now fly your way to Central City. Search the grid for a "Plains" marker. You will get the capsule!